The uptake of therapeutic peptides into cells following in vivo administration is not efficient. A substantial amount of peptides are removed extremely rapidly from the plasma by a variety of mechanisms including cellular uptake and metabolism, filtration by the kidney and urinary excretion or destruction by the proximal tubule cells of the kidney. Furthermore the lipid bilayer of the cell membrane presents a barrier to transport. Yet another disadvantage of these peptides is the need to administer them by injection because peptides in general possess very poor oral or nasal bioavailability.
The human immunodeficiency virus (HIV) produces a long gag polyprotein which is cleaved during viral budding into smaller proteins which have specific viral functions. This proteolytic cleavage is catalyzed by a specific HIV aspartyl protease enzyme (HIV PR). Small peptide fragments which conformationally resemble the HIV protease binding site but have the p1 site replaced by non-hydrolyzable structures act as inhibitors of the HIV protease and may be useful as therapeutic agents in AIDS.
A number of such protease inhibitors have been synthesized. See for example, Merck European Patent Application # EP 0 337 714 A, publications by Upjohn, Science 247:454-456 (1990), Roche European Patent Application EP 0 346 847 A2, Smith, Kline French International Patent Application WO 90/00399, Nature 343:90-92 (1990), Proc. Natl. Acad Sci.USA 86:9752-9756 (1989), Hoechst European patent Application EP 0 354 522 A1. All of these protease-inhibitors are peptides, and as peptides, are subject to a general problem of metabolic degradation and clearance which will prevent them from reaching the infected cell targets to bind HIV PR.
Protease inhibitory peptides, as well as other therapeutic peptides, lack targeting mechanisms in their native form, and on administration to an animal they become distributed globally. For that reason they may not effectively treat infected tissues. For example, macrophages are believed to be a major site of HIV infection, and inhibitory peptides by themselves do not target to the macrophage reservoir.
These problems can be overcome by preparing conjugates of therapeutic peptides which resist clearance and degradation, and which can be targeted to the cell.
It is therefore the object of the invention to overcome the problem of degradation and clearance of therapeutic peptides, including HIV protease inhibitory peptides, from the plasma. It is further the object of the invention to provide peptides comprising nonhydrolyzable groups capable of inhibiting HIV PR. Another object of this invention is to provide for prodrugs of protease inhibitory peptides which treat the site of viral infection effectively. Yet another object of this invention is to improve the oral and nasal bioavailability of therapeutic peptides.